1. Field of the Invention
The present invention pertains to electrical methods and devices for electronically detecting the presence of air (or other gas or vapor) inside a thermal inkjet printhead to sense whether an unfavorable printing condition exists. More specifically, the present invention relates to a detecting method and apparatus for sensing the presence of a non-collapsing bubble in a cell of a thermal inkjet printer, and activating a repriming circuit if the non-collapsing bubble is detected.
2. Discussion of Related Art
The advent of thermal inkjet printheads has brought affordability to high quality printing. Examples of thermal inkjet printheads are found in Drake et al, U.S. Pat. No. 4,789,425 and Drake et al U.S. Pat. No. 4,829,324. Thermal inkjet printing systems use thermal energy selectively produced by resistors located in capillary filled ink channels near channel terminating nozzles or orifices to vaporize momentarily the ink and form bubbles on demand. Each temporary bubble expels an ink droplet and propels it towards a recording medium. The printing system may be incorporated in either a carriage type printer or a pagewidth type printer. The carriage type printer generally has a relatively small printhead, containing the ink channels and nozzles. The printhead is attached to a disposable ink supply cartridge and the combined printhead and cartridge assembly is reciprocated to print one swath of information at a time on a stationarily held recording medium, such as paper. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath, so that the next printed swath will be contiguous therewith. The procedure is repeated until the entire page is printed. For an example of a cartridge type printer, refer to U.S. Pat. No. 4,571,599 to Rezanka. In contrast, the pagewidth printer has a stationary printhead having a length equal to or greater than the width of the paper. The paper is continually moved past the pagewidth printhead in a direction normal to the printhead length and at a constant speed during the printing process. Refer to U.S. Pat. No. 4,829,324 to Drake et al for an example of pagewidth printing.
U.S. Pat. No. 4,829,324 mentioned above discloses a printhead having one or more ink filled channels which are replenished by capillary action. A meniscus is formed at each nozzle to prevent ink from weeping therefrom. A resistor or heater is located in each channel upstream from the nozzles. Current pulses representative of data signals are applied to the resistors to momentarily vaporize the ink in contact therewith and form a bubble for each current pulse. Ink droplets are expelled from each nozzle by the growth of the bubbles which causes a quantity of ink to bulge from the nozzle and break off into a droplet at the beginning of the bubble collapse. The current pulses are shaped to prevent the meniscus from breaking up and receding too far into the channels, after each droplet is expelled. Various embodiments of linear arrays of thermal inkjet devices are shown, such as those having staggered linear arrays attached to the top and bottom of a heat sinking substrate for the purpose of obtaining a pagewidth printhead, and large arrays of printhead subunits butted against each other to form an array having the length of a pagewidth. Such arrangements may also be used for different colored inks to enable multi-colored printing.
However, during normal printing operations, a noncollapsible bubble of air or other has may appear inside the cells or channels of an inkjet head. Such bubbles typically result through desorption from the ink or ingestion of air. These non-collapsing bubbles are not to be confused with the normal collapsing bubbles which are required to expel ink droplets in normal operation. If a non-collapsing bubble is sufficiently large or close to a heating mechanism, printing quality will be adversely affected. If a bubble becomes sufficiently large, the cell will no longer be able to emit droplets and blank spaces or deletions will appear in the printed characters.
Typically, a repriming operation has been the means by which printing quality is restored. When a user perceived that printing quality had diminished, he or she could manually activate a repriming function. Thus, manual activation of the repriming function has the disadvantage that corrective action is only taken upon visually perceiving a reduction in printing quality.
As a remedy, machines can be designed to continually reprime at preset intervals. However, needless consumption of ink and time are but two of the disadvantages in such systems.
Isayama, U S. Pat. No. 4,518,974 and Nagashima, U.S. Pat. No. 4,625,220 both disclose piezoelectric-type inkjet printing devices which ar equipped with detection circuits which detect variations in voltage levels in the piezoelectric elements positioned adjacent to the ink chamber of a nozzle located in the printing head. The detecting devices of the Isayama and Nagashima patents discern different voltage levels in the piezoelectric elements when air bubbles are present in an adjacent nozzle than when the nozzle is filled solely with ink. The detection circuit taught by Isayama is a rather complicated one which detects an oscillating component of the voltage appearing between a pair of terminals of a piezoelectric element. The devices of Isayama and Nagashima are further complicated by the presence of a piezo detection transducer which exists in addition to the bubble-generating transducer. Since the systems of Isayama and Nagashima are used with piezoelectric transducers, these references do not teach or suggest the present invention.
Of course, when air bubbles are detected as being present in the cell or chamber of the printhead, an air bubble removing system should be activated. Air bubble removing systems are disclosed in, for example,. Yoshimura, U.S. Pat. No. 4,466,005 and Scardovi, U.S. Pat. No. 4,695,852.